JP3737230B2 - Color image processing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a color image processing apparatus that automatically determines whether characters in a document are chromatic or achromatic, and performs processing according to the determination result.
[0002]
[Prior art]
In image processing apparatuses such as digital color copiers and digital copiers, when reproducing an image in which characters and designs are mixed, in order to obtain a high-quality reproduced image, high-gradation processing is performed on the design. It is desirable to perform processing with an emphasis on resolution on characters. In order to realize such processing, it is necessary to separate the character area and the pattern area in the image with high accuracy. That is, for example, a color image that is provided with a means for detecting the hue of an original and a means for determining whether the original is a character area or a halftone area, and is reproduced in a black color when the original is an achromatic color and a character area. Although there is a processing device (see Japanese Patent Application Laid-Open No. 64-72662), black characters can be reproduced in high quality by reproducing them in black.
[0003]
However, it is not easy to detect the achromatic region due to a problem in the performance of the scanner, and many color characters are detected as black characters. On the other hand, if it is attempted to accurately determine the color character, the determination accuracy of the black character is lowered, and the black character is colored to deteriorate the image quality. Since black characters are frequently used in a document that is normally used, it is necessary to accurately determine black characters.For this reason, priority is given to the determination of black characters in the chromatic / achromatic color criteria. Next, color characters are determined.
[0004]
By the way, on a map or the like, there are many cases where a color itself has a meaning rather than a black character always having a black color. For example, purple / brown characters / line drawings are used. However, since it is difficult to distinguish them from black, they are determined to be black characters, which causes a significant deterioration in image quality. In particular, the topographic map published by the Geospatial Information Authority of Japan has contour lines drawn in brown lines, and when this is copied, there is a problem that most contour lines are output in black.
[0005]
In order to solve such a problem, for example, as described in JP-A-2-249365 (color image processing apparatus), means for determining an achromatic region of an input image, and an achromatic region It is necessary to improve the accuracy of chromatic / achromatic color determination by providing a signal correcting means (that is, when a chromatic color area is detected, the chromatic color area is not determined as an achromatic color area). .
[0006]
[Problems to be solved by the invention]
However, in the above-described conventional technology, it is necessary to consider the working accuracy of the scanner, the position correction of the line sensor, the difference in MTF characteristics of the RGB color components, and the increase in cost is inevitable.
[0007]
In addition, there is a color image processing apparatus in which when the color reading signal is classified into an achromatic color and a chromatic color, a region where the achromatic color and the chromatic color are classified is changed according to a mode set from the outside (Japanese Patent Laid-Open No. Hei. (See Japanese Patent No. 2-280573). In this apparatus, when the photo mode or the character mode is set, an area determined to be an intermediate color is assigned to a chromatic area when the photo mode is set, and to an achromatic area when the character mode is set. However, in this device, color characters are easily output as black characters in the character mode, and as a result, the purple characters and brown contour lines described above are increasingly reproduced in a single black color, which causes the above-mentioned problems. Still not resolved.
[0008]
The present invention has been made in consideration of the above circumstances,
An object of the present invention is to provide a color image processing apparatus that accurately determines color characters in a document.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, according to the first aspect of the present invention, means for reading a document as a digital image signal, color character detecting means for detecting a color character satisfying a predetermined condition from the image signal, and frequency detecting means for detecting the frequency, the frequency of the color character selects the first threshold value is higher than the predetermined value, when the frequency of the color character is lower than a predetermined value second threshold (> first threshold value ) And a color difference of the image signal when the selected first or second threshold value is compared with the color difference of the image signal, and a chromatic color when the color difference of the image signal exceeds the first or second threshold value. And determining means for determining that the color is achromatic when it does not exceed .
[0010]
The invention according to claim 2 is characterized in that the color character detecting means comprises means for detecting a character and means for detecting a color region under a predetermined condition.
[0011]
According to a third aspect of the present invention, the frequency detection means detects the frequency based on the total area of the document area and the total area of the color character and the detected area.
[0012]
According to a fourth aspect of the present invention, the frequency detecting means detects the frequency based on the total area of the character and the detected area and the total area of the color character and the detected area.
[0013]
According to a fifth aspect of the present invention, the frequency detection means performs a first calculation based on the total area of the document area and the total area of the character and the detected area, and the total area of the detected area as the character. In addition, the second calculation is performed based on the total area of the color character and the detected area, and the frequency is detected based on the first and second calculation results.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
<Example 1>
FIG. 1 shows the configuration of Embodiment 1 of the present invention. In the figure, 1 is a color character detection means, 2 is a frequency detection means for detecting the frequency of a color character, 3 is a parameter selection means for selecting a parameter according to the detected frequency, and 4 is a color using the selected parameter. A chromatic / achromatic determination means for determining a chromatic / achromatic region of image data.
[0015]
In this embodiment, first, a pre-scan is performed. An original is read using an image input device such as a scanner to obtain an image signal. The color character detection means 1 performs color character detection using this image signal.
[0016]
5A and 5B show the configuration of the color character detection means. In the configuration of (a), after the specific color detection unit 41 detects a specific color, the character detection unit 42 detects characters only in the area having the color. In the configuration of (b), the character detection is reversed. After performing the above, only the character of a specific color is detected for only the character area.
[0017]
FIG. 6 shows a specific color area detected by the specific color detection means 41. That is, for the RGB image signal, the RGB minimum value x = min (RGB) is in the range [a, b] (a and b are predetermined values), and the maximum value max (RGB) and the minimum value min of the primary expression RGB. The difference d of (RGB) is d <m1x + n1, d> m2x + n2.
When the condition is satisfied, it is detected as a specific color (m1, m2, n1, and n2 are predetermined constants).
[0018]
Also, as character detection means, for example, Ouchi, Imao, Yamada, IEICE Transactions Vo1. J75-D-2 No. 1 1992-01 “Image area separation method of mixed image of characters / patterns (halftone dots, photographs)” is used. That is, the character area has many pixels with high and low levels of density (hereinafter referred to as black pixels and white pixels), and these black pixels and white pixels are continuous at the edge portion. Character edges are detected based on the continuity of the black and white pixels. FIG. 8 shows the configuration of the edge region determination means.
[0019]
The ternary processing unit performs ternarization (white pixel <TH1, TH1 ≦ intermediate density pixel <TH2, TH2 ≦ black pixel) with two fixed threshold values (TH1, TH2) for the input image signal. Here, TH1 and TH2 are threshold values used for the ternary processing. For example, when the input data is represented by 256 gradations (0 is white) of 0 to 255, TH1 = 20 and TH2 = 80. Takes a value.
[0020]
In the continuity detecting unit, a portion where the black pixel and the white pixel after ternarization are continued is detected by pattern matching. An example of the pattern to be used is shown in FIG. Pixels matching these patterns are detected as black continuous pixels and white continuous pixels.
[0021]
The proximity detection unit determines whether the black continuous pixel and the white continuous pixel are in the vicinity. In a block having a size of 5 × 5 pixels, when one or more black continuous pixels and one or more white continuous pixels exist, the block is determined as an edge region, and otherwise, it is determined as a non-edge region. “1” is output in a pixel determined to be an edge region.
[0022]
Returning to FIG. 1, the frequency detection means 2 calculates the frequency of color characters in all images by dividing the area of the area detected as a color character by the total area of the image data. In accordance with this frequency value, the parameter selection means 3 selects a parameter to be output to the chromatic / achromatic determination means 4.
[0023]
Next, using the image data obtained during the main scan, the chromatic / achromatic determination unit 4 determines the chromatic / achromatic region of the image. The parameters used at this time are the parameters selected at the time of pre-scanning.
[0024]
FIG. 7 shows the configuration of the chromatic / achromatic determination means. As shown in FIG. 7, the circuits 51, 52, and 53 for calculating the absolute value of the color difference obtain the absolute values of the color differences of R, G, and B, and output max (| RG |, | G−B |, | B−R |) and a predetermined threshold th are compared with each other by a comparator 55, and when the predetermined threshold is not exceeded, it is determined that the color is achromatic, and when it exceeds, the color is determined to be chromatic. When the color character frequency is high, the parameter selection unit 3 selects th1 as the threshold value, and when the color character frequency is low, the parameter selection unit 3 selects th2 (> th1) as the threshold value. As a result, it is possible to accurately determine a brown character or the like erroneously determined as a black character as a chromatic color.
[0025]
Note that the output of the chromatic / achromatic determining means is given to, for example, a UCR circuit constituting the color image processing apparatus. That is, when it is determined to be chromatic, control is performed to output Y, M, C, and K. When it is determined to be achromatic, the output of Y, M, and C is set to 0 and only K is output. Control to be black (monochrome).
[0026]
In addition, the chromatic / achromatic determination means described above is determined in units of pixels. The determined pixels are changed to a block composed of a plurality of pixels (for example, 8 × 8 pixels), and chromatic / achromatic is determined for each block. You may do it.
[0027]
<Example 2>
FIG. 2 shows a configuration of the second embodiment of the present invention. The color character detection unit according to the second embodiment includes the specific color detection unit 21, the character detection unit 22, and the AND gate 23 described with reference to FIG. First, pre-scanning is performed, and an original is read using an image input device such as a scanner to obtain an image signal. The specific color detection unit 21 performs specific color detection using the image signal, and the character detection unit 22 performs character detection. The AND gate 23 outputs a color character when it is a specific color and a character is detected.
[0028]
The frequency detection unit 24 calculates the frequency of the color characters in all the images by dividing the area of the area detected as the color character by the total area of the image data. The parameter selection unit 25 selects a parameter to be output to the chromatic / achromatic determination unit 26 according to the frequency value.
[0029]
Next, a character is detected by the character detection unit 22 using the image data obtained during the main scan, and a chromatic / achromatic determination is performed by the chromatic / achromatic determination unit 26 using parameters preselected in the pre-scan. Do. That is, when the character detection unit 22 detects a character and the chromatic / achromatic determination unit 26 determines a chromatic color, the AND gate 27 outputs a chromatic color character, and the character detection unit 22 detects the character. When the chromatic / achromatic determination means 26 determines an achromatic color, the AND gate 27 outputs an achromatic color character. In this embodiment, since the common character detection means 22 can be used at the time of pre-scanning and main scanning, the configuration is simplified and there is a cost advantage.
[0030]
<Example 3>
FIG. 3 shows the configuration of Embodiment 3 of the present invention. The difference from the second embodiment is that the output of the character detection unit 22 is directly input to the frequency detection unit 24. The specific color detection unit 21 and the character detection unit 22 function in the same manner as in the second embodiment. Then, at the time of pre-scanning, the frequency detection unit 24 divides the area of the region detected as a color character that is the output of the AND gate 23 by the total area of the region determined as a character by the character detection unit 22, thereby obtaining an image. The frequency of color characters in the character area is calculated. The parameter selection unit 25 selects a parameter to be output to the chromatic / achromatic determination unit 26 according to the frequency value.
[0031]
Next, using the image data obtained at the time of the main scan, the character detection unit 22 detects characters, and the chromatic / achromatic determination unit 26 performs chromatic / achromatic determination using parameters preselected in the pre-scan. Do. Similarly to the second embodiment, when the character detection unit 22 detects a character and the chromatic / achromatic determination unit 26 determines a chromatic color, the AND gate 27 outputs a chromatic color character, and the character detection unit 22. Detects a character and the chromatic / achromatic determination means 26 determines an achromatic color, the AND gate 27 outputs an achromatic color character.
[0032]
<Example 4>
FIG. 4 shows a configuration of Embodiment 4 of the present invention. The frequency detection means of the fourth embodiment includes a first calculation means 29, a second calculation means 28, and two comparators 30 and 31. Since other configurations are the same as those of the third embodiment, description thereof is omitted.
[0033]
At the time of pre-scanning, the first calculation unit 29 performs a calculation to divide the area of the character area detected by the character detection unit 22 by the total area of the image data. The second calculation means 28 performs an operation of dividing the area of the area detected as a color character by the area of the character area detected by the character detection means 22. The comparators 31 and 30 decode the first calculation result and the second calculation result to a 2-bit frequency by comparing them with the threshold values th3 and th4, respectively. In accordance with the frequency value, the parameter selection unit 25 selects a parameter. For example, when the first calculation result and the second calculation result are both greater than th3 and th4, a threshold value (FIG. 7) for determining a chromatic color is selected as a parameter, and the first calculation result is greater than th3, When the second calculation result is less than th4, a threshold value for determining an achromatic color is selected as a parameter.
[0034]
Characters are detected by the character detection unit using the image data obtained during the main scan, and chromatic / achromatic determination is performed by the chromatic / achromatic determination unit using the parameters selected in advance by the pre-scan. That is, chromatic color characters and achromatic color characters are detected based on the character detection result and the chromatic / achromatic determination result.
[0035]
【The invention's effect】
As described above, according to the first and second aspects of the invention, the determination criteria for chromatic and achromatic colors are controlled in accordance with the frequency of color characters and black characters included in the document. It is possible to accurately and automatically perform chromatic / achromatic determination processing that matches the characteristics.
[0036]
According to the third, fourth, and fifth aspects of the present invention, since the appearance frequency of the color characters is normalized by the size of the original, the ratio of the characters, etc., the color characters can be detected with high accuracy.
[Brief description of the drawings]
FIG. 1 shows a configuration of Embodiment 1 of the present invention.
FIG. 2 shows a configuration of Embodiment 2 of the present invention.
FIG. 3 shows a configuration of Embodiment 3 of the present invention.
FIG. 4 shows a configuration of Embodiment 4 of the present invention.
FIGS. 5A and 5B show the configuration of color character detection means. FIG.
FIG. 6 shows a specific color area detected by a specific color detection unit;
FIG. 7 shows a configuration of chromatic / achromatic determination means.
FIG. 8 shows a configuration of an edge region determination unit.
FIG. 9 shows an example of a pattern for detecting the continuity of black pixels and white pixels.
[Explanation of symbols]
1 color character detection means 2 frequency detection means 3 parameter selection means 4 chromatic / achromatic determination means

Claims (5)

Means for reading a document as a digital image signal; color character detection means for detecting a color character satisfying a predetermined condition from the image signal; frequency detection means for detecting the appearance frequency of the color character; and the frequency of the color character. when higher than the predetermined value and selecting the first threshold value, and selecting means frequency of the color character is to select a time less than a predetermined value second threshold (> first threshold value), the first or the selected A threshold value of 2 and a color difference of the image signal, a determination unit that determines a chromatic color when the color difference of the image signal exceeds the first or second threshold, and determines an achromatic color when the color difference does not exceed the first or second threshold value. A color image processing apparatus comprising:   2. A color image processing apparatus according to claim 1, wherein said color character detecting means comprises means for detecting a character and means for detecting a color area of a predetermined condition.   The color image processing apparatus according to claim 1, wherein the frequency detection unit detects the frequency based on a total area of the document area and a total area of the color character and the detected area.   3. The color image processing according to claim 1, wherein the frequency detection unit detects the frequency based on a total area of the character and the detected area and a total area of the color character and the detected area. apparatus.   The frequency detection means performs a first calculation based on the total area of the document area and the total area of the characters and the detected area, and detects the total area of the characters and the detected area and the color characters and the detected areas. 3. The color image processing apparatus according to claim 1, wherein a second calculation is performed based on the total area, and the frequency is detected based on the first and second calculation results.

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